This invention relates to compounds and liquid crystal compositions containing them which are useful in electooptical and non-linear optical applications.
Liquid crystal (LC) displays are now nearly ubiquitous in our culture, being used in both monochrome and color displays in a variety of products from watches to automobile gauges and from road signs to computer displays. It is most desirable that monochrome displays are simply black and white with no particular cast of color. Similarly, it is imperative for quality color displays that all colors be transmitted equally well. If a display is less transmissive for one wavelength compared to another, the display will not show true colors and will be less marketable than a display showing true colors.
LC displays rely on the birefringence (xcex94n) of the LC, i.e., the difference in refractive indices between different orientations of the LC. Birefringence, xcex94n=nexe2x88x92n0, where ne is the index of refraction along the extraordinary axis of a birefringent material (parallel to the optic axis) and n0 is the index of refraction its ordinary axis (perpendicular to the optic axis). The optimal thickness of an LC cell such that it behaves as a half-wave plate, to maximize contrast and true color transmission, at a given wavelength is proportional to the birefringence. The optimal birefringence for a fixed pathlength, i.e., thickness of LC, increases with increasing wavelength as shown in FIG. 1. In contrast, birefringence of a given material generally decreases as a function of increasing wavelength (FIG. 1). The change in birefringence of a material as a function of wavelength is called birefringence dispersion. (Herein, the term xe2x80x9cpositive birefringence dispersionxe2x80x9d is used for birefringence that decreases with increasing wavelength and xe2x80x9cnegative birefringence dispersionxe2x80x9d is used for birefringence dispersion that decreases with increasing wavelength.) Thus, if birefringence of an LC cell is optimized for transmission at one wavelength by optimization of cell thickness, it will not be the optimal birefringence at a second wavelength and as a consequence light transmission through the cell at the second wavelength will be lower.
Typically, in designing an LC device, a compromise is made by setting cell thickness for optimal transmission of a wavelength in the middle of the operational wavelength range (i.e., at the design wavelength). For LC devices used in the visible, cell thickness is chosen to optimize transmission of green light, giving a cell less than optimal, but useful, transmission in the red and blue. Such a cell has a slight yellow or green cast.
If the birefringence dispersion of an LC material were negative (increasing in slope as a function of increasing wavelength), cells made from this material would exhibit significantly less chromatic behavior. In general, LC materials, i.e. mesogenic compositions, which exhibit a lower positive (including zero) or negative birefringence dispersion than existing materials will be useful for decreasing the chromatic behavior of LC displays and related electrooptical devices. Such mesogenic materials will be useful in optical filters with improved color balance, larger free spectral range, maintaining high resolution with fewer filter stages and in tunable Fabry-Perot filters using liquid crystal spatial light modulators (SLMs).
Furthermore, ferroelectric liquid crystals (FLCs) used in displays often have quite high birefringence requiring the use of thin cells. When thin LC cells are used, small variances in cell thickness can have a significant effect on the cell""s optical properties. For example, a 0.1 xcexcm variance in thickness of a cell that is 1.1 xcexcm thick results in a xc2x19% difference in transmission, while the same variance in a thicker 1.9 xcexcm cell results in only a xc2x15% difference. Thinner LC cells also tend to suffer from non-uniform spacing, which can lead to shorts. Environmental contamination of LC cells, for example by inclusion of dust and other contaminants, has a more severe effect on thinner cells. Designs using thicker cells, for more stability, easier manufacturing and lower cost, require LC materials with generally lower birefringence (compared to presently available materials). There is a general need in the art for LC materials, particularly FLC materials, with decreased birefringence.
Ferroelectric liquid crystals (FLCs) are true fluids possessing thermodynamically stable polar order. As the liquid crystal cools from a normal isotropic liquid to a crystalline state, it passes through a series of mesogenic phases of increasing order. A typical phase sequence includes several phases, of which only the tilted smectic C* (S*c) phase possesses the thermodynamically stable polar order necessary to exhibit a net dipole moment. In the S*c phase the molecules self-assemble into layers, with the long axis of the molecules coherently tilted with respect to the layer normal. The single polar axis of the phase is normal to the tilt plane. For most such FLCs, a spontaneous macroscopic dipole density or spontaneous ferroelectric polarization P along the polar axis is easily measurable.
The ferroelectric nature of a C* phase affords a very strong coupling of the molecular orientation with external fields, leading to a high contrast electro-optic light valve with fast response relative to the well known nematic devices currently in use. The complicating factor of the C* helix was solved with the invention of the Surface Stabilized Ferroelectric Liquid Crystal (SSFLC) light valve. In the SSFLC geometry, the helix is spontaneously unwound due to surface interactions with bounding glass plates. In this case, when the director prefers a parallel orientation with respect to the surface plates, two states are allowed. In one state the molecules tilt right by tilt angle xcex8, while in the other state they tilt left. In both cases, the ferroelectric polarization vector is pointing normal to the title plane (normal to the surface of the glass plates).
Due to the birefringence of FLC molecules, the two states have different optical characteristics. When the tilt angle xcex8=22.5xc2x0, and the thickness of the cell is tuned correctly relative to the birefringence, then the cell behaves as a half wave plate, and can be aligned between crossed polarizers such that one state gives good transmission, while the other state shows good extinction, giving rise to the desired electro-optic effect.
SSFLC cells show very high contrast (1,500:1 demonstrated), low switching energy, bistability, high resolution (xe2x89xa1107 pixels/cm2 demonstrated, 108 pixels/cm2 possible) and other performance characteristics which make it particularly attractive for many optoelectronic applications.
Compounds which self-assemble into the smectic C phase are often termed C phase mesogens. While there is currently no detailed understanding of the relationship between molecular structure and the occurrence of LC phases, empirically, C phase mesogens generally possess a rigid core separating two xe2x80x9cfloppyxe2x80x9d tails. The tails of chiral and achiral mesogens can include a variety of chemical functionalities, but components of commercial mixtures often have one or two alkyl or alkoxy tails. The tails often have similar lengths, and both are typically longer than four carbons. Many compounds of this type also exhibit a nematic phase. For C* mesogens generally one of the tails will possess one or more tetrahedral stereocenters.
In order to be useful in the many types of devices of interest, FLC materials must possess properties never achievable in a single compound, but the stable temperature range and other material parameters can in general be tuned by mixing components. Commercial LC mixtures are generally composed of at least eight components. FLC mixtures generally contain two types of components: 1) A smectic C host, designed to afford the required temperature range and other standard LC properties; and 2) Chiral components designed to induce ferroelectric polarization and produce fast switching or other desirable properties (e.g., tilt angle adjustment)in the FLC film. FLC mixture may also contain additional actiral components that adjust other desirable FLC properties.
Birefringence refers to the property of a liquid crystal to interact more strongly with light along one LC axis than along another LC axis. As discussed above, most LCs are made of a core with extensive electron delocalization, to which one or two tails are attached to help orient the molecules, give a dipole moment or polarization or confer other desirable properties on the molecule. Typical LC are rod-shaped with the majority of the pi-electron delocalization along the long or extraordinary axis (also referred to as the director). As a consequence the extraordinary axis of LCs have the higher index of refraction, so their birefringence xcex94n=nexe2x88x92n0 is positive. Birefrinence of a liquid crystal at a given wavelength is:             Δ      ⁢              xe2x80x83            ⁢      n        =                  G        ⁢                  (          T          )                    ⁢              xe2x80x83            ⁢                                    λ            2                    ⁢                      λ                          *              2                                                            λ            2                    -                      λ                          *              2                                            ,
where xcex94n is the birefringence at a given wavelength, G is a constant, T is the temperature, xcex is the particular wavelength, xcex* is the mean resonance frequency which can be calculated given the spectrum of a material or the its birefringence at several wavelengths. See: S.-T. W (1986) Phys. Rev. A 33:1270; S.-T. W (1987) Opt. Eng. 26:120; S.-T. W, C.-S. W (1989) J. Appl. Phys. 66:5297; S.-T. W et al. (1993) Opt. Eng. 32:1775. As the wavelength of interest moves away from xcex*, the birefringence decreases asymptotically until in the infrared, the birefrinence is relatively constant (except near IR absorbencies). There is, however, a large amount of birefringence dispersion in the visible spectrum. This is particularly true if xcex* is close to the visible region so that xcex2xe2x88x92xcex*2 is small. When the birefringence of the typical LC is higher at short wavelengths than at longer wavelengths, optimization of LC cells as half-waveplates at a given wavelength generally require the opposite behavior of birefringence as a function of wavelength. FLC cell half-wave plates in particular require:   d  =      λ          2      ⁢              xe2x80x83            ⁢      Δ      ⁢              xe2x80x83            ⁢      n      
where a wavelength (xcex) of about 500 nm is usually chosen as the optimal for LC cells (for applications in the visible). As indicated in FIG. 1, this thickness is then not optimal for all wavelengths of visible light due to the birefringence dispersion.
FIG. 2 shows transmission (measured and calculated) at different wavelengths for cells with three different FLC materials (a standard mixture and two theoretical mixtures), normalized for 100% transmission of 500 nm light. The first measured transmission (solid line) is ZLI 3654 which has typical birefringence behavior with a xcex* of 217 nm. With this mixture, only about half of the 400 nm light and about 70% of the 700 nm is transmitted. A cell using this material has a noticeable greenish cast. The second, a calculated transmission (dotted line) is that based on use of a theoretical material in which the birefringence is invariant with changing wavelength. A cell using such a material is calculated to transmit about 83% of 400 nm light and about 83% of 700 nm light. Such a cell would have much truer color, particularly in the blue, compared to the standard FLC cell. The third, another calculated transmission (dashed line) is that based on use of a theoretical material in which the birefringence dispersion is negative, with the absolute value of the proportional change as a function of wavelength the same as for the standard LC mixture. A cell using such a material is calculated to transmit very nearly 100% of blue light and 92% of red light, resulting in a cell with quite true colors.
The present invention relates in one aspect to low birefringence mesogens or to mesogens having anomalous birefringence dispersion. As used herein, xe2x80x9canomalousxe2x80x9d refers to birefringence dispersion atypical for liquid crystal material, either exhibiting zero (as illustrated in FIG. 2 dotted line) or negative birefringence dispersion (increasing with increasing wavelength) (as illustrated in FIG. 2 dashed line), or significantly less positive birefringence dispersion compared to known LC materials. Compounds of this invention can be mesogens or can be employed as components in mesogenic compositions to lower birefringence or to lower birefringence dispersion.
Mesogenic materials of the present invention are chiral nonracemic and achiral materials having mesogenic phases (LC phases) useful in electrooptical devices, including chiral and achiral tilted smectic phase materials (particularly smectic C and smectic A materials) and nematic phase materials. Mesogenic materials of this invention include those that are ferroelectric liquid crystals (FLCs), nematic liquid crystals, and materials useful in SSFLC, electroclinic and DHF devices.
While the discussion of anomalous birefringence dispersion herein has focused on FLC""s used in SSFLC devices, other electrooptic devices employing LCs, such as nematic cells will also benefit from the use of LC""s with low or negative birefringence dispersion (and more generally from LCs with low or negative birefringence). The mechanism of FLC and nematic cells differ, but in both cases optimization of cell thickness depends on the wavelength of light being transmitted. In nematic cells some efforts have been made to make cells achromatic. For example, a combination of at least two polymer retarder films can be employed as compensators to give light that is reasonably achromatic (T. Scheffer, J. Nehring (1995) SID Seminar Lecture Notes, Vol. 1, M2). However, the use of such external means of chromaticity compensation can be detrimental to contrast ratio or viewing angle of the device. Thus, nematic liquid crystals with little or no birefringence dispersion would be quite beneficial.
Development of methods for creation of organic thin films with large "khgr"(2) is a problem of great interest due to the potential utility of such films in the fabrication of fast integrated electro-optic (EO) modulators. Such modulators are hybrid devices wherein the organic material must work in concert with semiconductor integrated circuits. For this application the material design and synthesis task involves three key considerations: 1) Molecules with large molecular second order susceptibility "khgr" must be created; 2) The molecules must be assembled into a material with the correct supermolecular stereochemistry to afford the required bulk "khgr"(2); and 3) This material must be integrated with a semiconductor devicexe2x80x94a key process requiring supermolecular stereocontrol on a more global level.
Early in the development of chiral smectic FLC chemistry and physics, the spontaneous thermodynamically stable polar supermolecular structure exhibited by these anisotropic liquids suggested their potential utility in second order nonlinear optics applications. This work, however, showed that FLCs known at the time, such as DOBAMBC, exhibited values of "khgr"(2) too small to be useful (deffxcx9c0.001 pm/V for SHG from 1064 nm light). Efforts directed toward design of FLCs with increased "khgr"(2) provided materials with demonstrated EO coefficients on the order of 1 pm/V for modulation of 633 nm light at 100 MHz and d coefficients on the order of 5 pm/V for SHG (Arnett et al. (1995) xe2x80x9cTechnique for Measuring Electronic-Based Electro-Optic Coefficients for Ferroelectric Liquid Crystalsxe2x80x9d in Thin Films for Integrated Optics Applications, Wessels et al. (eds), Materials Research Society (Pittsburg, Pa.); Walba et al. (1991) Ferroelectrics 121:247; Schmitt, K. et al. (1993) Liq. Cryst. 14:1735). Still, further increases in the magnitude of "khgr"(2) are required, since EO coefficients on the order of 50 pm/V are desirable for fast integrated optics applications (Walba, D. M. (1995) Science 270:250). Achieving such a large increase in "khgr"(2), however, seems problematical in FLCs since functional arrays with large molecular susceptibility xcex2 are typically xe2x80x9clong,xe2x80x9d and tend to orient along the director when incorporated into traditional thermotropic liquid crystal (LC) structures. Since the FLC polar axis is normal to the director, small values of "khgr"(2) result as observed for DOBAMBC.
Achieving large "khgr"(2) in FLCs involves orientation of xe2x80x9clarge xcex2xe2x80x9d functional arrays, typically possessing two rings and a conjugating spacer unit, along the polar axis with a high degree of supermolecular stereocontrol (Williams, D. J. (1984) Angew. Chem. Int. Ed. Engl. 23:690; Quantum Chemical Computational calculations of Nonlinear Susceptibilities of Organic Materials; Bredas, J. L. et al.(eds.); Special Issue of Mol. Cryst. Liq. Cryst. Sci. Technol. Sect. B, Gordon and Breach (1994) 6(3-4):135; Kanis et al. (1994) Chem. Rev. 94:195; Meyers et al. (1994) J. Am. Chem. Soc. 116:10703.) Excellent supermolecular stereocontrol is indeed achievable in FLCs (on the order of 60% polar excess has been demonstrated as evidenced by ferroelectric polarization measurements), and xe2x80x9clarge xcex2xe2x80x9d functional arrays are easily incorporated into LC structures (Kobayashi et al. (1990) Mol. Cryst. Liq. Cryst. Letts. 7:105; Fouquey et al. (1987) J. Chem. Soc. Chem. Comm. 1424; Berdague et al. (1993) Liq. Cryst. 14:667; Ikeda et al. (1993) Nature 361:428; Sasaki et al (1994) J. Am. Chem. Soc. 116:625). However, in all know cases such functional arrays orient along the liquid crystal director {circumflex over (n)}, while the FLC polar axis is normal to the director.
This invention relates in a second aspect to LC compounds for NLO applications. While an LC mesogen is not required for NLO FLCs (doping of achiral C phase hosts with appropriate non-mesogenic quests delivers the required supermolecular stereocontrol) mesogenicity is desirable since for NLO applications achieving the highest possible concentration of NLO active units is advantageous. Certain compounds of this invention that have negative birefringence also exhibit NLO properties. Herein we demonstrate an approach for achieving large "khgr"(2) in FLCs with examples of a class of chiral smectic materials demonstrating orientation of large xcex2 NLO chromophores along the polar axis.
WO 92/20058 (Walba et al.) published Nov. 12, 1992 relates to ferroelectric liquid crystal materials for nonlinear optics applications. Certain of the compounds reported therein are monomers for the side-by-side dimer materials of this invention. WO 92/20058 takes priority from U.S. patent application Ser. No. 07/690,633 filed Apr. 24, 1991 (now abandoned). U.S. patent application Ser. No. 08/137,093, filed Oct. 18, 1993, (now allowed) is the U.S. national stage application of WO 92/20058. WO 92/20058, U.S. Ser. Nos. 07/690,633 and 08/137,093 (U.S. Pat. No. 5,543,078) are incorporated in their entirety by reference herein.
The following U.S. patents provide general descriptions of LC""s for electrooptical applications, including FLCs: U.S. Pat. Nos. 5,051,506, 5,061,814, 5,167,855, 5,178,791, 5,178,793, 5,180,520, 5,271,864, 5,380,460, 5,422,037, 5,453,218, and 5,457,235. These patents are incorporated by reference in their entirety herein and provide methods of synthesis for a variety of LC materials, including methods of synthesis of a variety of LC cores and chiral and achiral LC tails that are used in the compounds of this invention. These patents also provide a general description of the properties of LC materials for electrooptical applications, particularly those for use in SSFLC, electroclinic, DHF and nematic devices.
This invention provides mesogenic compositions which exhibit anomalous dispersion. More specifically this invention provides LC compositions useful as birefringent materials in electrooptic devices which exhibit zero or low negatively sloped birefringence dispersion (e.g., exhibiting positive birefringence dispersion significantly lower than that of currently available LC compositions) or more preferably positively sloped birefringence dispersion in which birefringence of the material increases with wavelength. As a means to this end, the invention provides compounds useful as components of LC compositions which exhibit negative birefringence where n0 is higher than ne. These compounds can be doped into LC compositions having typical positive birefringence dispersion to reduce that dispersion. LC compositions with reduced birefringence dispersion can be used to make LC cells and other electrooptical devices having decreased chromaticity. Preferred compounds of this invention with negative xcex94n are those that mix with available FLC and/or nematic mixtures with minimal suppression of the desired mesogenic phases.
Additionally, this invention provides LC electrooptic devices having optical retardance substantially independent of wavelength. The invention particularly relates to LC electrooptic devices for use in the visible.
In a second aspect certain of the dimers of this invention have useful NLO properties.
Most generally, the compounds of this invention are dimers of LC-like compounds in which the monomers are linked to each other through a high birefringence moiety (dimerization linker). The LC monomers consist of an LC core and one or two tail groups. Preferred monomers for this invention have low birefringence in comparison to the birefringence of the monomer linking moiety. The monomers are linked to each other such that the relatively low birefringence groups, with little conjugation, comprise the long axis of the molecule and the high birefringence linking moiety is substantially perpendicular to that low birefringence long axis. This long axis preferably aligns with the director. These dimers, have more extensive conjugation and high birefringence along the ordinary axis and as a consequence exhibit negative birefringence. The dimers have normal positive birefringence dispersion, to have birefringence that is lower in absolute value at longer wavelengths. But since they have negative birefringence, their birefringence actually increases (i.e., goes less negative) as wavelength increases.
A particularly interesting subset of compounds of this invention are those having negative birefringence and which exhibit strong absorption bands near, but not in the visible. These materials exhibit negative birefringence and birefringence dispersion that slopes steeply in the blue portion of the spectrum. The xcex* of a material can be adjusted by changing functional groups in the high birefringence linker between monomers. Substitution with functional groups including among others, nitro groups, azo groups, Shiff bases, ketones, sulfonates, thiols, and amines can be employed to adjust absorption spectrum of the compounds of this invention.
When mixed into standard LC materials with positive birefringence dispersion, compounds of this invention with negative birefringence result in mixtures having reduced birefringence dispersion or negative birefringence dispersion.
Compounds of this invention include those of formula I: 
where
a is 0 or 1 and A is selected from the group consisting of a xe2x80x94Cxe2x95x90Cxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94Cxe2x95x90Cxe2x80x94Cxe2x95x90Cxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94Cxe2x95x90Cxe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94Nxe2x95x90Nxe2x80x94, xe2x80x94Nxe2x95x90NOxe2x80x94, and a xe2x80x94HCxe2x95x90Nxe2x80x94 group;
b1-b4, independently of one another, are 0 or 1 and B1-B4, independently of one another, are selected from the group consisting of xe2x80x94Cxe2x95x90Cxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OOCxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COSxe2x80x94, xe2x80x94SOCxe2x80x94, xe2x80x94CHxe2x95x90CHCOOxe2x80x94, xe2x80x94OOCCHxe2x95x90CHxe2x80x94, xe2x80x94CHxe2x95x90CHCOSxe2x80x94, and xe2x80x94SOCCHxe2x95x90CHxe2x80x94;
d1 and d2, independently of one another, are 0 or 1 and D1 and D2, independently of one another, are selected from the group consisting of xe2x80x94Cxe2x95x90Cxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OOCxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COSxe2x80x94, xe2x80x94SOCxe2x80x94, xe2x80x94CHxe2x95x90CHCOOxe2x80x94, xe2x80x94OOCCHxe2x95x90CHxe2x80x94, xe2x80x94CHxe2x95x90CHCOSxe2x80x94, and xe2x80x94SOCCHxe2x95x90CHxe2x80x94;
e1 and e2, independently of one another, are 0 or 1 and E1 and E2, independently of one another, are selected from the group consisting of xe2x80x94Cxe2x95x90Cxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OOCxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COSxe2x80x94, xe2x80x94SOCxe2x80x94, xe2x80x94CHxe2x95x90CHCOOxe2x80x94, xe2x80x94OOCCH xe2x95x90CHxe2x80x94, xe2x80x94CHxe2x95x90CHCOSxe2x80x94, and xe2x80x94SOCCHxe2x95x90CHxe2x80x94;
six-membered aromatic rings E and F, independently of one another, are phenyl rings or phenyl rings in which one or two of the carbon atoms of the ring are replaced with nitrogen atoms and wherein the carbon atoms of the phenyl or nitrogen-containing phenyl rings can be substituted with a halogen, CN, NO2, alkyl, alkenyl, alkynyl, haloalkyl, haloalkenyl, or haloalkynyl group (preferred halogens being fluorines);
Y1-Y4, substituents on rings E and F, independently of one another, are selected from the group consisting of H, halogen, CN, alkyl, alkenyl, alkynyl, haloalkyl, haloalkenyl, or haloalkynyl group (preferred halogens being fluorines) wherein one or more non-neighboring CH2 groups in the substituent can be substituted with an O, or S (e.g., giving alkoxy, ether, thioether or related groups) or with a SiRARB group, where RA and RB are small alkyl or alkenyl groups having from 1 to about 6 carbon atoms, with the proviso that any ring position of aromatic rings E or F that is a nitrogen is not substituted with any of the Y1-Y4;
x and z, independently of one another are 0 or 1, and X and Z, independently of one another, are selected from the group consisting of electron acceptor groups, electron donor groups, H, halogen, NO2, xe2x80x94Cxe2x95x90Cxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OOCxe2x80x94, xe2x80x94COxe2x80x94, O, S, xe2x80x94COSxe2x80x94, xe2x80x94SCOxe2x80x94, CN, NH, NCH3 (more generally NRxe2x80x2, where Rxe2x80x2 is a small alkyl having 1 to about 3 carbon atoms), NHCO, NCH3CO (more generally NRxe2x80x2CO, where Rxe2x80x2 is a small alkyl having 1 to about 3 carbon atoms), SO, and SO2, with the proviso that any ring position of aromatic rings E or F that is a nitrogen is not substituted with any X or Z, and R5 and R6, independently of one another, are selected from the group consisting of H, halogen, CN, alkyl, alkenyl, alkynyl, haloalkyl, haloalkenyl, or haloalkynyl group (preferred halogens being fluorines) wherein one or more non-neighboring CH2 groups in the substituent can be substituted with an O, or S (e.g., giving alkoxy, ether, thioether or related groups) or with a SiRARB group, where RA and RB are small alkyl or alkenyl groups having from 1 to about 6 carbon atoms, dependent upon the X or Z group, R5 and/or R6 may be absent; and
M1-M4, independently of one another, are core moieties having from one to four aromatic or non-aromatic rings, optionally separated by up to three linking groups F1-F3 as in formula:
xe2x80x94[N1]n1xe2x80x94[F1]f1xe2x80x94[N2]n2xe2x80x94[F2]f2xe2x80x94[N3]n3xe2x80x94[F3]f3xe2x80x94[N4]n4xe2x80x94
xe2x80x83where
f1-f4, independently of one another, are 0 or 1, F1-F4, independently of one another, are selected from the group consisting of xe2x80x94Cxe2x95x90Cxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OOCxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COSxe2x80x94, xe2x80x94SOCxe2x80x94, xe2x80x94CHxe2x95x90CHCOOxe2x80x94, xe2x80x94OOCCHxe2x95x90CHxe2x80x94, xe2x80x94CHxe2x95x90CHCOSxe2x80x94, and xe2x80x94SOCCHxe2x95x90CHxe2x80x94; and
n1-n4, independently of one another, are 0 or 1, and N1-N4 are selected from the group consisting of aromatic rings having one or two six-member and/or five-membered aromatic rings, which may be fused or non-fused ring systems, or monocyclic or bicyclic alkyl and alkenyl non-aromatic rings having from 5 to about 12 ring carbon atoms wherein in each ring of N1-N4, one or more of the ring carbons can be substituted with a halogen, CN, small alkyl, alkenyl or alkynyl group having from 1 to about 3 carbon atoms or small halogenated alkyl, halogenated alkenyl or halogenated alkynyl having from 1 to about 3 carbon atoms preferred halogens being fluorines), in each ring of N1-N4 that is aromatic, one or two of the ring carbons can be replaced with a nitrogen (N), in each ring of N1-N4 that is non-aromatic, one or two non-neighboring CH2 groups can be replaced with an oxygen; and
R1, R2, R3, and R4, independently of one another, are selected from the group consisting of linear, branched or cyclic alkyl, alkenyl or alkynyl groups having from 1 to about 20 carbon atoms wherein one or more CH2 groups can be optionally substituted with one or more halogens, or CN groups, or in which one or more non-neighboring CH2 groups can be replaced with an oxygen, a sulfur, or a substituted silyl group, Si(RA)(Rxe2x80x2), in which RA and RB, independently, are alkyl alkenyl, alkynyl, haloalkyl, haloalkenyl or haloalkynyl groups, preferably those having from 1 to about 6 carbon atoms (preferred halogens being fluorines).
R1, R2, R3, and R4 groups can be chiral nonracemic groups or achiral groups dependent upon the desired application of the compound. A subset of R1-R4 groups are fully or partially fluorinated alkyl, alkenyl or alkynyl groups, designated by RF. Preferred R1-R4 include those that have about 6 to about 12 carbon atoms. Preferred Y1-Y4 that are alkyl, alkenyl, alkynyl or halogenated derivatives thereof are those that have from 1 to about 6 carbon atoms. Preferred R5 and R6 that are alkyl, alkenyl, alkynyl or halogenated derivatives thereof are those having from 1 to about 6 carbon atoms.
Preferred N1-N4 are aromatic rings having one six-member aromatic ring and monocyclic or bicyclic alkyl and alkenyl non-aromatic rings having from 5 to about 12 ring carbon atoms. More preferred monocyclic non-aromatic rings are cyclohexane and cyclohexane rings.
X and Z groups can include electron donor groups and/or electron acceptor groups as defined herein below.
In general herein, unless otherwise stated alkyl, alkenyl and alkynyl groups can contain linear, branched or cyclic portions, may be fully or partially halogenated, one or more carbons of the group may be substituted with halogen or CN, and one or more non-neighboring CH2 groups can be replaced with an O, S or Si(RA)(RB), in which RA and RB, independently, are alkyl, alkenyl, alkynyl, haloalkyl, haloalkenyl or haloalkynyl groups, preferably those having from 1 to about 6 carbon atoms (preferred halogens being fluorines).
In the most general sense, the six member aromatic rings E and F in formula I can be replace with other aromatic systems, including 5-member aromatic rings and aromatic systems having one or two 5- or 6-member aromatic rings, wherein the aromatic ring system can be fused or non-fused ring system.
Compounds of this invention also include those of formula II and III: 
where a, A, b1, b2, B1, B2, d1, d2, D1, D2, e1, e2, E1, E2, Y1-Y4, x, z. X, Z, M1, M2 and R1-R6 are as defined for formula I; 
where a, A, b1, b2, B1, B2, d1, d2, D1, D2, e1, e2, E1, E2, Y1-Y4, M1, M2 and R1-R4 are as defined for formula I and;
X and Z, independently of one another except as specifically stated herein, are selected from one of the following:
(1) the group consisting of H, an electron donor or an electron acceptor, with the proviso that when one of X or Z is an electron donor, the other of X or Z is an electron acceptor; or
(2) the group consisting of H, halogen, CN, small alkyl, alkenyl or alkynyl groups having from 1 to about 3 carbon atoms, or small halogenated alkyl, alkenyl or alkynyl group having from 1 to about 3 carbon atoms with the proviso that any ring position of aromatic rings E or F that is a nitrogen does not carry a substituent.
R1, R2, R3 and R4 groups can be chiral nonracemic groups or achiral groups dependent upon the desired application of the compound. A particular subset of R1-R4 groups are those that are fully or partially fluorinated, designated by the variable RF.
X and Z electron donor groups include any grouping known in the art to be an electron donor, for example any grouping causing activation of an aromatic ring relative to benzene in an electrophilic aromatic substitution reaction. Electron donors include groups in which the group atom connected to the aromatic ring is less electronegative than a halogen and where that atom possesses a lone pair able to interact with the aromatic ring in a resonance sense. Electron donors include: ORxe2x80x3, NRxe2x80x3Rxe2x80x2xe2x80x3, NRxe2x80x3CORxe2x80x2xe2x80x3, and OCORxe2x80x3, where Rxe2x80x3 and Rxe2x80x2xe2x80x3, independently of one another, are H or an alkyl having from 1 to about 6 carbon atoms (preferably having from 1 to 3 carbon atoms and most preferably methyl). More preferred electron donors are NRxe2x80x3Rxe2x80x2xe2x80x3, with N(CH3)2 being most preferred.
A particular subset of X and Z groups are those where one of X or Z is an electron donor and the other of X or Z is an electron acceptor.
X and Z electron acceptor groups include any grouping known in the art to be an electron acceptor, for example any grouping causing deactivation of an aromatic ring relative to benzene in an electrophilic aromatic substitution reaction. Electron acceptors include, among others, halogens, CN, (CN)Cxe2x95x90C(CN)2, CORxe2x80x3, CO2Rxe2x80x3, CONRxe2x80x3Rxe2x80x2xe2x80x3, SO2Rxe2x80x3, SO2CF3 and NO2 where Rxe2x80x3 and Rxe2x80x2xe2x80x3, independently of one another (and independent of Rxe2x80x3 and Rxe2x80x2xe2x80x3 groups of any electron donor), are H or alkyl or haloalkyl (preferably a fluoroalkyl) having from 1 to about 6 carbon atoms (preferably having 1 to 3 carbon atoms and most preferably methyl), except that in the group SORxe2x80x3, Rxe2x80x3 cannot be H. NO2 is preferred over halogen and CN for obtaining large molecular xcex2. The (CN)Cxe2x95x90C(CN)2, SO2CF3 and NO2 are generally more preferred acceptors. SO2CF3 and NO2 are more preferred acceptors for ferroelectric liquid crystal materials. The NHCOCH3 grouping can be an acceptor if the lone pair on nitrogen is unable to interact with the aromatic ring in a resonance sense.
Compounds of this invention also include those of formula IV: 
wherein a, b1, b2, d1, d2, e1, e2, A, B1, B2, D1, D2, E1, E2, Ring F, and Ring F, X, Z, Y1-Y4, M1, M2, R1 and R2 are as defined for formula III; and those of formula V: 
wherein a, b1, b2, e1, e2, A, B1, B2, E1, E2, Ring E and Ring F, X, Z, Y1-Y4, M1, M2, R1 and R2 are as defined for foitnula II and wherein b3, b4, e3, and e4 are 0 or 1, B3 and B4, independently of one another and B1 and B2, can be the same groups as defined for B1 and B2, E3 and E4, independently of one another and E1 and E2, can be the same groups as defined for E1 and E2, M3 and M4, independently of one another and M1 and M2, can be the same groups as defined for M1 and M2, R3 and R4, independently of one another and R1 and R2, can be the same groups as defined for R1 and R2.
In a more specific embodiment compounds of this include those of formula VI: 
wherein
c1 and c2, independently of one another, are 0 or 1 and C1 and C2, independently of one another, are selected from the group consisting of xe2x80x94Cxe2x95x90Cxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OOCxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COSxe2x80x94, xe2x80x94SCOxe2x80x94, xe2x80x94CHxe2x95x90CHCOOxe2x80x94, xe2x80x94OOCCHxe2x95x90CHxe2x80x94, xe2x80x94CHxe2x95x90CHCOSxe2x80x94 and xe2x80x94SOCHxe2x95x90CHxe2x80x94; and
six-membered aromatic rings G, H, I and J, independently of one another, are 1,4-phenyl rings or 1,4-phenyl rings in which one or two of the carbon atoms of the ring are replaced with nitrogen atoms and in which carbons of the phenyl rings or nitrogen-containing phenyl rings can be substituted with halogens, CN, NO2 or small alkyl, alkenyl, alkynyl, haloalkyl, haloalkenyl or haloalkynyl groups having from 1 to about 3 carbon atoms and all other variables are as defmed in formula III.
M1 and M2 and (M3 and M4) moieties of the compounds of this invention include, but are not limited to those in which one or more of the N1-N4 are
a 1,4-phenyl,
a 1,4-phenyl in which one or two of the ring carbon atoms are replaced with nitrogens,
a 1,4-phenyl in which one or more of the carbon atoms of the ring are substituted with a halogen, CN, small alkyl, alkenyl or alkynyl group having from 1 to about 3 carbon atoms, or small halogenated alkyl, alkenyl or alkynyl group having from 1 to about 3 carbon atoms,
a 1,4-phenyl in which one or two of the ring carbon atoms are replaced with nitrogens and wherein one or more of the ring carbons are substituted with a halogen, CN group, small alkyl, alkenyl or alkynyl group having from 1 to about 3 carbon atoms, or small halogenated alkyl, alkenyl or alkynyl group having from 1 to about 3 carbon atoms,
a 1,4-cyclohexyl or 1,4-cyclohexenyl group,
a 1,4-cyclohexyl or 1,4-cyclohexenyl group in which one or two of the non-neighboring CH2 groups are replaced with an oxygen,
a 1,4-cyclohexyl or 1,4-cyclohexenyl group in which one or more of the ring carbons are substituted with a halogen or CN group,
a bicyclic alkyl or bicyclic alkenyl group having from 5 to about 12 carbon atoms,
a bicyclic alkyl or bicyclic alkenyl group having from 5 to about 12 carbon atoms in which one or two of the non-neighboring ring CH2 groups are replaced with an oxygen atom; and
a bicyclic alkyl or bicyclic alkenyl group having from 5 to about 12 carbon atoms, in which one or more of the ring carbons are substituted with a halogen or CN group.
M1 and M2 (and M3 and M4) groups of this invention include those in which one or more of N1-N4 are bicyclic [2,2,n] alkyl or alkenyl ring groups comprising a cyclohexyl or cyclohexenyl ring where n is an integer from 1 to about 6 wherein the bicyclic ring is optionally substituted with one or more halogens or CN groups.
Compounds of this invention also include those in which M1, M2, M3, and M4 are 1,4-cyclohexyl or 1,4-cyclohexenyl group, particularly those in which the cyclohexyl or cyclohexenyl group is in the trans configuration.
xe2x80x9cAxe2x80x9d groups are conjugating linker groups between the LC monomers. When a is 0 in formulas I-VI, then there is a single bond linking the rings of the dimerization link, preferred dimerization linkers when a is 0 are those containing at least one aromatic ring, such as biphenyl groups, phenylpyridine groups or phenylpyrimidine groups. Preferred xe2x80x9cAxe2x80x9d groups are xe2x80x94Cxe2x89xa1Cxe2x80x94, trans xe2x80x94Cxe2x95x90Cxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94Cxe2x89xa1Cxe2x80x94 and xe2x80x94Nxe2x95x90Nxe2x80x94.
Preferred for NLO applications are compounds of this invention of formulas I-VI in which X is an electron donor or acceptor and Z is an electron acceptor or donor. More preferred electron acceptors are xe2x80x94NH2 and N(CH3)2. Preferred electron donors are CN or NO2.
Preferred achiral tails are those in which E1-E4 are O or S or those in which e1-e4 are 0.
In general, any chiral nonracemic LC tail group that is known to be useful in the preparation of FLC""s or FLC dopant materials can be employed as a chiral tail group in the compounds of this invention. U.S. Pat. Nos. 5,051,506, 5,061,814, 5,167,855, 5,178,791, 5,178,793, 5,180,520, 5,271,864, 5,380,460, 5,422,037, 5,453,218, and 5,457,235, provide exemplary chiral (as well as achiral) tail groups for FLC, DHF and nematic LCs. Specifically, chiral tails of this invention include chiral 1-methylalkoxy groups, chiral 2,3-dihaloalkoxy groups, chiral 2-halo-3-methyl alkoxy and ester tails.